Apparatus for performing a ground line continuity test at reduced voltage



Nov. 24, 1964 c. s. CONRAD APPARATUS POR PERFORMING A GROUND LINECONTINUITY TEST AT REDUCED VOLTAGE Filed Oct. 3. 1960 CHESTER S. CONRADA; ATTORNEY United Statesg Patent O APPARATUS FOR PERFRMING A GROUNDLINE CNTINUETY TEST AT REDUCED VOLTAGE Chester S. Conrad, Fairmont, W.Va., assigner to Consolidation Coal Company, Pittsburgh, Pa., acorporation oi Pennsylvania Filed ct. 3, 1969, Ser. No. 59,877 3 Claims.(Cl. .3M-Sil lThe present invention relates to a method and equipmentfor testing the ground line of* grounded electrical equipment forycontinuity vand more particularly to a method and equipment for testinghigh voltage A.C. ground lines by conditioning the high voltageequipment to respond to a lowered test voltage and thereafterirnpressing the lowered test voltage on the ground line and observingthe response.

In electrical equipment, particularly high voltage A.C. equipment thatis utilized in coal mines, it is important that the equipment beadequately grounded for operational safety. Power is transmitted to mostA.C. powered mobile mining equipment, 'such as mining machines,

shuttle cars, drills, or the like by a trailing cable which is connectedat one end to the particular piece of equipment andat` the other end tothe terminal of the load center whichy supplies the power. The trailingcable includes three power conductors each of which conducts one A.C.phase and a ground conductor or ground line which is secured to theframe of the particular mining machine involved and which is grounded atthe load center. Thus, the ground line grounds the frame of the miningmachine at a point adjacent the load center.

To insure that the mining machine is adequately grounded at all times,the continuity of the ground line from the machine to the load centermust be periodically checked. Until the present invention, the checkingof a ground line for continuity could be a relatively hazardousoperation. The standard ground line continuity check for A.C. poweredmining equipmentwas to intentionally cause a full line voltage groundfault at the mining machine in order to observe Whether the circuitbreaker at the load center was tripped or opened in response to theground fault. lf the circuit breaker was opened, then there was groundcontinuity from the machine to the load center and the machine wasproperly grounded.

When the ground continuity test was accomplished in the foregoingmanner, the frame of the mining machine was subjected to full linevoltage; that is the voltage between the line and neutral. The machineoperator per* forming the ground check was seated on the machine whilethe machine was subjected to thisV full line voltage by an intentionalground fault. So long as the operator did not touch the door or passagesor other source of ground while seated on the machine he was perfectlysafe. However, if he touched ground while the ground check was beingperformed, there was the lpossibility thathe 3,158,806 Patented Nov. 24,i954 ICC served while a ground fault at a lowered test voltage isintentionally imposed upon the mining equipment. lf the load centerresponds, as by opening a circuit breaker, to the lowered voltage groundfault, then ground continuityfrom the equipment to the load center isassured. Broadly stated then, the present invention consists of (1)conditioning the load center to respond to a gorund fault of reducedvoltage, (2) providing an intentional ground fault of reduced voltage onthe mining equipment, and (3) observing the response at the load centerto the intentional ground fault.

In order to accomplish conditioning of the load center, test blocks iareinstalled at strategic locations within the load center. These testblocks form certain strategic wiring junctions withinthe load center.The test blocks are equipped with sockets which receive test jacks whichare similar to the jacks utilized on telephone switchboards. With thetest jacks removed, the test blocks cause the wiring of the load centerto remain as conventional load centers are wired. When the test jacksare inserted, cer tain circuits of the load center are opened and othercir-4 cuits are closed so that the load center is substantially rewiredto respond to a reduced ground fault voltage.

ln laccomplishing the ground continuity check of the present invention,the test jacks are inserted in the load center to condition the loadcenter to respond to a reduced ground fault voltage. rl`he mobile miningequipment is equipped with a test transformer which transforms the linevoltage to a substantially reduced test voltage. The test transformer isthen grounded to the frame of the mining equipment to create Ianintentional ground fault at reduced test voltage. The response at theload center is observed and if the circuit breaker within the loadcenter is tripped it is apparent that there is ground continuity fromthe equipment to the load center. However, if the circuit breakerremains closed, corrective action must be taken to determine the openingin the ground line. Once the test is completed, the test jacks areremoved and the load center is returned to its normal wiring conditionand again responds only to high voltage ground faults as is desired innormal operation. n

A standard load center usually consists of an incoming line with linevoltage impressed thereon. The incoming line directs current to a mainbus within the load center through a main circuit breaker. The main busfeeds a series of branch circuits which may power individual pieces ofmining equipment.` Each of the branch circuits has a branch circuitbreaker between the main bus and the corresponding piece of r.equipmentso that if there is troubleon one of the 'branch circuits, the branchcircuit Y breaker will be tripped open while the main circuit breakerwould be subjected to full line voltage and be electrocuted.

Further, if someone walking in the lmine passage happened to touch themining machine while the ground check was being performed, he too mightmeet with a fatal accident.

The presentinvention is directed to accomplishing a ground linecontinuity check without exposingQ personnel to the hazards of full linevoltage ground checks. This invention contemplates a novel method andnovel equipment for accomplishing Aa ground continuity test at voltagessubstantially lower than full line voltage so that the hazards yof fullline voltage ground faults will not be present during'the testingoperation. ln practicing the present invention, the load center fromwhich the mining equipment is powered is modified to respond to a groundfault substantially lower than a full line voltage ground fault.l/henfso modified, the load center may be ob` feeding the main bus andthe remaining branch circuits on which there is no trouble may remain inoperation. As aprecaution against inadvertently leaving the test jacksmounted within the load center and thereby causing damage to the loadcenter, a time delay relay is actuated whenthe test jacks are installedto practice the present invention. ll`his time delay relay is set toclose a circuitk to the main circuit breaker tripping relay toy open themain circuit ybreaker after a predetermined time period. Thispredetermined time period is designed to be long enough yto permit theground-test to be accomplished. When the test jacks are removed, thetime rdelay relay is no longer actuated. 1n the event that a groundcheck is accomplished and the test jacksfare inadvertently lett inplace, then the time delay relay will cause tripping of the main circuitbreaker within the load center which, through loss of power to allequipment fed by that load center, will indicate to operatingspersonnelthat the test jacks have not been removed and that they should beremoved before thetload center is put back into operation. v

With theforegoing.considerations in mind, it is a primary object of thepresent invention to provide a safe method of accomplishing a groundline continuity check.

It is another object of this invention to provide simple, inexpensivemodiiications to load center equipment and mobile mining equipment toperfom safe ground line continuity tests on the equipment.

It is another object of this invention to provide a test which isdesigned to be performed within a predetermined time period and topermit modification of the load center for test purposes only during thepredetermined time period.

These and other objectives achieved by this invention will becomeapparent as this description proceeds in conjunction with theaccompanying drawing.

The drawing is a schematic circuit diagram of the load center and apiece of mining equipment modilied to practice the present invention.

Referring to the drawing, a load center l and a power consuming piece ofmobile equipment 12 such as a mining machine, a shuttle car, a drill, orthe like are connected by a trailing cable 14.

ln describing the structure of the load center l0 and the mobileequipment 12, the standard unmodified wiring configurations will iirstbe described in detail. The modications of the standard equipment toaccomplish the present invention will then be described in detailfollowed by the method of performing a completely safe ground continuitycheck by the present invention.

The load center l@ contains a transformer 16 that has delta-connectedprimary coils 18 which receive 4,160 volts, 3 phase A.C. from an outsidesource. The deltaconnected secondary coils 2t) of transformer 16 produce480 volts, 3 phase A.C.

Current from secondary coils Z0 is conducted through a three-phaseconductor 22 to a main bus Z4 through a main circuit breaker 26. Maincircuit breaker 26 is equipped with a magnetic protective relay 28 and athermal protective relay 30 each of which are shown schematically in thedrawing. As is conventional in delta-connected transformers, a Yconnected zigzag transformer 32 is associated with conductor 22 toestablish a neutral at 33. The neutral of zigzag transformer 32 isconnected to a ground stud 34 through a ground resistor 36 and theactuating coil 3? of a protective relay having contactors 40. Theconnecting of the zigzag transformer to the ground stud 34 through aground resistor is completely conventional.

The relay having actuating coil 38 and contactors 46, connected by themechanical linkage d2, is utilized to close a circuit to the maincircuit breaker trip relay coil 44 which opens circuit breaker 26through mechanical linkage 45 in the event of a ground fault. Since therelay having coil 3S and contactors 40 controls the load center maincircuit breaker 26, it is set to respond to only a relatively largeground fault current so that ordinarily the branch circuit breakers areopened before the main circuit breaker 26.

Trip relay coil 44 has lines i6 to conduct current from a localcontrol-voltage source to relay coil 44. The contactors 40 close onepossible circuit to the trip relay coil e4, in the event of'a groundfault, to open circuit breaker 26. The contactors 4 8 may also close asecond circuit to actuate trip relay coil 44. The function of contactors48 will be discussed in greater detail at a later point in thisspeciiication.

The main bus 24 may have several branch circuits extending therefrom.One three-phase branch conductor u is shown in the drawing. Branchconductor 50 has a branch circuit breaker 52 disposed therein. Circuitbreaker 52 has a magnetic protective relay 54 and a thermal protectiverelay S6 which are indicated schematically. The branch circuit breaker52 also has a ground fault trip relay coil 53 which opens the circuitbreaker 52 through mechanical linkage 59 when trip relay coil 58 isactuated by current passing therethrough.

Cil

In order to detect ground faults in the branch circuits, a ring typecurrent transformer 60 is provided on conductor 50. The coil 62 oftransformer 6i) is connected by lines 64 and 66 respectively to the triprelay 58. In the event that there is a ground fault within the branchcircuit fed by conductor S0, unbalanced current Will liow Within theindividual phase conductors of three-phase conduotor Si) therebycreating a ground fault current in transformer 6d. This ground faultcurrent in coil 62 will be conducted to relay coil 5S through lines 64.and 66. The actuation of relay coil 58 causes the circuit breaker 52 toopen in response to the ground fault. Relay coil 58 is designed to bemore responsive to a ground fault than is relay coil 33 controlling themain circuit breaker. Accordingly, a smaller ground fault will causecircuit breaker 52 to open than will cause main circuit breaker 26 toopen.

Conductor Sti is joined to a three phase trailing cable terminal 68formed on the load center 10. A ground line '72 within the load centerl0 connects the trailing cable terminal 68 to the ground stud 34 on anormal, unmodiiied load center.

The mining equipmentlZ to be powered by branch circuit conduit 5ft has athree phase trailing cable terminal 74 ormed thereon. The three phasetrailing cable 14 joins the load center terminal 63 with the machineterminal 7d. Trailing cable ld has three insulated phase conductorscontained therein. Cable i4 also has a ground conductor or ground lineinsulated from the phase conductors and solidly connected through loadcenter terminal 68 to ground line 72 within the load center.

The mining equipment l2 has a motor 76 that has phase conductors 78, and82 connecting it with the trailing cable terminal 74. Circuit breakers86 are included in each of the phase conductors 7S, Si) and 82. Theground line in trailing cable 14 is connected through terminal '74 to aground line SS within the mobile equipment l2 and ground line S8 isanchored solidly to the vehicle frame at 90.

As described thus far, the load center 10 and the mobile miningequipment 12 are completely conventional. Hitherto, in order to performa ground continuity check an intentional ground fault was created bygrounding one of the phase conductors 78, Sil or S2 to the frame of themachine l2 at full line voltage to see whether the circuit breaker 52would trip in response to the ground fault. The hazards of creating aground fault at full line voltage on the mining equipment have beendiscussed above.

In order to practice the present invention, the previously describedload center l@ is modified by adding test blocks 92 and 9d and timedelay relay 96. The test block 92 contains normally closed ground linecontactors 98 and wil which when closed provide a continuous ground line72 from the trailing cable terminal 68 to the ground stud 34. The testblock 92 also contains groundtest contactors N2 and litri. Ground testcontactor 102 is normally open and is provided in a ground testconductor 106 which connects test block 92 to test block 94. Ground testcontactor ldd is located in ground test conductor 68 which also connectstest block 92 to test block 94.

Within test block 94, conductor 106 contains a ground test contacter lltwhich is normally open. The ground test conductor 166 joins line 64which connects to trip relay coil 53. Ground test conductor lltitcontains a ground test contactor i12 within the test block 94. Testcontactor i12 is normally open. The conductor 10S joins line 66 whichconnects to test relay coil 5S.

The test block 4- also contains transformer disconnect contactor M4disposed in line 64- and transformer disconnect contactor M6 disposed inline 66. Contactors El@ and 116 are normally closed so that trip relaycoil 58 is normally connected to the coil 62 of transformer 6d. Whentest jacks are inserted, however, the normally closed contactors 114 and116 are opened to disconnect the relay coil y from transformer 62. Inorder to render the transformer 60 ineffective when contactors 114 and116 are open, a normally open transformer shorting contactor 118 isclosed to directly connect lines 64 and 66 to short circuit the coil 62.f

The test block 92 also controls the time delay relay 96 which controlsthe contractors 4S through mechanical linkage 120. Time delay relay 96has lines 122 and 124 which connect it to a source of electrical currentnot shown. The source of electrical current may be the same source towhich lines 46 that control the main. circuit breaker trip relay coil 44are connected. ln serieswith the time delay relay 96 is a time delayactuating coutactor 126 disposed within the test block 92. Contacter 126is normally opened so that no current can liow through time delay relay96.

The foregoing modications to the load center 1e by the addition of testblocks 92, 94 and time delay relay 96 allow the load center 10 to beconditioned for a response to a low Voltage test ground fault. Each ofthe test blocks 92 `and 94 are adapted to receive test jacks such as areutilized on telephone switchboards. So long als the test jacks are notinserted within test blocks 92 and 94 the normally closed contactors 93and 100`within the test block 92 and the normally closed contactors 114and 116 Within the test block 94 remain closed. Likewise, so long as thetest jacks are not inserted the normally open contactcrs 102, 11h-4 and126 within test block 92 and the normally open contactors 110, 112, and118 Within thetest block 94 remain open and the load center isconventional and unmodified. When, however, the test,

jacks are inserted in the test blocks 92 and 94 the normally opencontactors are closed and the normally closed contactors are opened sothat the load center is substantially rewired to respond to a lower testvoltage ground fault as will be described. It will be appreciated thatwhile the test blocks and test jacks aire a convenient methodwofaccomplishing the modication of the yload center for tes-t purposes,similar modification could be accomplished by utilizing toggle switchesor other positionable electrical controls to condition the load center10 to respond to a lower test voltage as will .be described.

The mining equipment 12 must be only slightly moditied to produce aground fault at a lowered ground faultl test voltage. To accomplish thismodilication, taps 123, 130 and 132 are secured to phase conductors 78,31D and 82 respectively. Taps 128, 13) and 132 are Y connected to theprimary coil of a test transformer 134. The secondary, delta-connectedcoil 138 of transformer 134 is unloaded. The Y connected primary coil136 of transformer 134 produces a neutral point 14? in the circuit. Thetransformer primary coil 136 lis tapped .at a rpoint removed from theneutral point 140 `so that a test voltage having a peak of from to 50volts may be obtained through Vthe tap 142. A ground test line 144 issecured to the tapand anchored to the vehicle frame at 146. A normallyopen ground, test switch 15@ may be closed to provide a ground faultfrom tap 142 to anchor 146 when desired. t

Under normal equipment operating conditions, the

ground test switch 15d remains open. The test jacks are not insertedwithin the test blocks 92 and 94 so that the load center operatesnormally. lf a ground fault should occur, `the unbalanced currentstlowing in the individual phase conductors of three phase conductor Silwould cause a current to be conducted through tripl relay 53 whichwould, in turn, open circuit breaker 52.

ln order to perform a ground continuity check according to the presentinvention, the test jacks (not shown) are inserted vinto test blocks 92and 94. When the test jacks are inserted in test block 92, the normallyclosed contactors. 93 andk llilii are opened so that there is no,

, longer a direct ground line from trailing Vcable terminal 68 to groundstud 34, but rather, ground line 72 is broken.

Normally open contactors 194 and 102 are closed. The

closing of contactors 104 and 1&2 creates a conductive path from loadcenter terminal 68 through ground line '72, contactor 104, conducto-r108, to the test block 94. There is also a conductive path from groundstud 34 through ground line 72, contacter 102, conductor 106 to testblock 94.

With the test jack inserted in test block 92, the contactor 126 isclosed and current is conducted to time delay relay 96. VTime delayrelay 96 is set to keep line 46 open for approximately five minutesafter the rst passage of current into relay 96. After approximately iiveminutes of continuous current passage through time delay relay 96, itcloses the contactors 48 .through mechanical linkage thereby causingcircuitbreaker26 to trip and opening the conductor 22 from transformer16 to the main bus 24. Under normal testing conditions, the test jackswill be removed from the test block 92 before the time delay relay 96ever has an opportunity to close contactons 48 and trip circuit breaker26.

The insertion of test jacks (not shown) into test block 94 closes thenormally open contactors 110, 112 and 118 and opens thenormally closedcontactors 114 and 11,6. The opening of contactors 114 and 116 and thekclosing of contactor 118 isolates the `trip relay. 58 from thetransformer coil 62. The closing of contactors 110 and 112 completes acircuit from conductor 72 through contactor 104 intest block 92, throughconductor 108, through contacter 112 in test block 94, through linef66,through trip relay '55,fthrough line 64, through contactor 110, throughline 166, and through contactor 102 to ground line 72.

After the test jacks have been inserted iny test blocks 92 and`94, theground test may be performed.k The test is initiated when the groundtest switch on the mining machine 12 is closed so that a peak groundtest voltage offrom 30 to 50 volts passes from transformer 134 to theframe of the machine 12. This creates an artificial test ground fault atthe machine and the current from the artificial ground fault can owthrough the conductive path of ground liney 88, machine trailing cableterminal 74, the ground line of trailing cable 14, and the terminal 68of the load center 10. The ground lineof the trailing cable 14 is iixedto ground line 72 within the load center terminal 68. Accordingly, theconductive path from the machine frame to terminal 68 will becontinuedby ground line 72, contactor 104, the conductor 108, contactor 112, line66, into relay coil 58 and through line 6d, contacterllilrconductorltld, contacter 102', ground line 'l2 to ground stud 34.

lf there is a continuous ground line, the relay coil 58 will .beactuated when a test groundr fault current how-s vthrough relay 58 bythe path set forth in the preceding paragraph, thereby tripping circuitbreaker 52. The tripping of circuit breaker 52 will indicate to anobserver atv the load center that there is a continuous ground line andthe test jacks may thenV be removed from the test blocks. 92 and 94 topermit the load center to resume normal operations.

in the event that the test jacks are inadvertently left in the testblocks 92 and 94, approximately tive minutes after their insertion thetime delay relay coil 96 will close contactors 4S thereby actuatingrelay 44 andtripping circuit brealter 26. When main circuit breaker26`is tripped, the resulting loss of power in the load center willindicate that the test jacks have not been removed.

Because kthe ring type transformer 60 operates on the unbalanced currentflowing in conductor 50 when an accidental ground faul-t occurs in thebranch fed by conductor 5d, the current within coil 62 is relativelysmall even with a full vline voltage ground fault and the current actingupon relay coil 58 in normal operation is, therefore, relatively small.Accordingly, if the test voltage ground fault of 30 to 50 vol-ts weregrounded tothe arcanos conductor St) would not be suicient to actuatethe relay S3 and thereby indicate a continuous ground from the equipmentback to the load center. For test purposes, it is necessary toprecondition the load center so that relay coil 58 receives the entirereduced ground fault test voltage rather than the transformed unbalancedcurrent as it does in an accidentally occurring ground fault.

It will be noted that with only 30 to 50 volts creating the test groundfault, there is no danger to operating personnel in performing theground check. At the same time, the modifications to the load center andto the mining equipment are relatively simple and inexpensive so thatthey may be accomplished with little or no difiiculty.

According to the provisions of the patent statutes, I have explained theprinciple, preferred construction, and mode of operation of my inventionand have illustrated and described what l now consider to represent itsbest embodiment, However, I desire to have it understood that, withinthe scope of the appended claims, the invention may be practicedotherwise than as specically illustrated and described.

l claim:

l. In an electrical power system including a source of current availablein a load center at a predetermined line voltage, circuit meansincluding a circuit breaker and conductors to conduct said current fromsaid source to a consuming device, a circuit breaker trip coilmechanically connected to said circuit breaker, a ground line connectingsaid consuming device to a ground within said load center in order toground said consuming device, and a current transformer electricallyconnected to said conductors and to said circuit breaker trip coil toexcite said trip coil upon the occurrence of a ground fault to operatesaid circuit breaker to thereby open said circuit breaker, theimprovement comprising a continuity testing means for said ground line,`said vtesting means including means to selectively disconnect saidcurrent transformer from said circuit breaker trip coil, means toselectively connect said ground line to said ground through said tripcoil within said load center so that current flowing in said ground linelows through said trip coil, and test transformer means in said circuitmeans and on said consuming device to selectively provide a ground faulttest current to said ground line at a voltage substantially lower thansaid line voltage.

2. In an electrical power system having a load center with a source ofA.C. electrical current, circuit means including a circuit breaker andconductors to control the flow of current at line voltage from saidsource to consuming equipment, a circuit breaker trip coil mechanicallyconnected to said circuit breaker, a ground line connecting a groundwithin said load center and the lframe of said consuming equipment, anda current transformer electrically connected to said conductors and tosaid circuit breaker trip coil to excite said trip coil upon theoccurrence of a ground fault to operate said circuit breaker to therebyopen said circuit breaker, the improvement comprising a continuitytesting means for said ground line, said testing means including meansto selectively disconnect said current transformer from said circuit gj,ce breaker trip coil, means to selectively connect said ground line tosaid ground through said trip coil Within said load center so thatcurrent owing in said ground line flows through said trip coil, and testtransformer means mounted on said consuming equipment and having primarycoils electrically connected to said conductor, a low voltage tap onsaid test transformer means primary coils, a test switch connected tosaid loW voltage tap and said consuming equipment frame and selectivelyclosable to impress a ground fault test current upon said ground line ata voltage substantially lower than said line voltage.

3. ln an electrical power system including a load center having a sourceof three phase A.C. at a predetermined line voltage, circuit meansincluding first three phase conducting means to conduct current fromsaid source to a main three phase bus within said load center, a maincircuit breaker in said rst three phase conducting means, second threephase conducting means to conduct current from said main bus toconsuming mobile equipment, a branch circuit breaker in said secondthree phase conducting means, a circuit breaker trip coil mechanicallyconnected to said branch circuit breaker, a ground line connecting aground Within said load center to said consuming mobile equipment, tirstrelay coil means mechanically connected to said main circuit breaker andpowered by a control voltage source to open said main circuit breakerwhen actuated, and current transformer means electrically connected tosaid branch circuit breaker trip coil and to said second three phaseconducting means, said current transformer means operable upon theoccurrence of a ground fault to operate said circuit breaker trip coilto open said branch circuit breaker, the improvement comprising acontinuity testing means for said ground line, said testing meansincluding means to selectively disconnect said current transformer meansfrom said branch circuit breaker trip coil, means to selectively connect said ground line to said ground through said branch circuit breakertrip coil Within said load center so that current flowing in said groundline flows through said branch circuit breaker trip coil, time delaymeans including a time delay relay and contactors to selectively connectsaid irst relay coil means to said control voltage source in order toactuate said rst relay coil means and thereby open said main circuitbreaker after a predetermined elapsed time period, said time delay meansconstructed and arranged to leave said control voltage sourcedisconnected from said tirst relay coil means at all times prior to thetermination of said elapsed time period Without said rst relay coilmeans being actuated, and test transformer means in said circuit meansmounted on said consuming mobile equipment and connected to said secondthree phase conducting means to selectively provide a ground fault testcurrent to said ground line at a voltage substantially lower than saidline voltage.

References tCited in the le of this patent UNlTED STATES PATENTS2,610,840 Baughman Aug. 13, 1935 2,478,147 Wilson Aug. 2, 1949 2,447,625Astin Aug. 24, 1948

1. IN AN ELECTRICAL POWER SYSTEM INCLUDING A SOURCE OF CURRENT AVAILABLEIN A LOAD CENTER AT A PREDETERMINED LINE VOLTAGE, CIRCUIT MEANSINCLUDING A CIRCUIT BREAKER AND CONDUCTORS TO CONDUCT SAID CURRENT FROMSAID SOURCE TO A CONSUMING DEVICE, A CIRCUIT BREAKER TRIP COILMECHANICALLY CONNECTED TO SAID CIRCUIT BREAKER, A GROUND LINE CONNECTINGSAID CONSUMING DEVICE TO A GROUND WITHIN SAID LOAD CENTER IN ORDER TOGROUND SAID CONSUMING DEVICE, AND A CURRENT TRANSFORMER ELECTRICALLYCONNECTED TO SAID CONDUCTORS AND TO SAID CIRCUIT BREAKER TRIP COIL TOEXCITE SAID TRIP COIL UPON THE OCCURRENCE OF A GROUND FAULT TO OPERATESAID CIRCUIT BREAKER TO THEREBY OPEN SAID CIRCUIT BREAKER, THEIMPROVEMENT COMPRISING A CONTINUITY TESTING MEANS FOR SAID GROUND LINE,SAID TESTING MEANS INCLUDING MEANS TO SELECTIVELY DISCONNECT SAIDCURRENT TRANSFORMER FROM SAID CIRCUIT BREAKER TRIP COIL, MEANS TOSELECTIVELY CONNECT SAID GROUND LINE TO SAID GROUND THROUGH SAID TRIPCOIL WITHIN SAID LOAD CENTER SO THAT CURRENT FLOWING IN SAID GROUND LINEFLOWS THROUGH SAID TRIP COIL, AND TEST TRANSFORMER MEANS IN SAID CIRCUITMEANS AND ON SAID CONSUMING DEVICE TO SELECTIVELY PROVIDE A GROUND FAULTTEST CURRENT TO SAID GROUND LINE AT A VOLTAGE SUBSTANTIALLY LOWER THANSAID LINE VOLTAGE.